I. INTRODUCTION

Among the federal provisions under the supervision of the Environmental Protection Agency (“EPA”), the Clean Air Act (“CAA”) has arguably been the most impactful on its target, air pollution.¹ Passed in 1970, the CAA has improved American air quality exponentially, resulting in better health and quality of life.² However, despite the significant improvements, there are rumblings among climate experts that the nation may be in store for a backslide.³ Climate change is the EPA’s elephant in the room, and its consequences have already forced the EPA to reconsider its enforcement of statutes like the CAA.⁴ In 2005, 42 U.S.C. § 7619 created the “exceptional event” rule, which provides a temporary exclusion from National Ambient Air Quality Standards (“NAAQS”) when an unpreventable or uncontrollable event results in significant exceedance of NAAQS.⁵ In 2016, the EPA updated its enforcement of the exceptional event rule, with a focus on four areas of environmental concern: transported pollution, forest fires, ozone, and wind.⁶

The exceptional event rule is a critical agency tool meant to ensure that data collected from the NAAQS monitoring stations is accurate and unaffected by statistically influential outlying events.⁷ It also ensures that states are not unfairly regulated when factors outside their control push them beyond the NAAQS or their State Implementation Plan.⁸ For example, down-wind states have reasonably relied upon the exceptional event rule for pollutants that originated from forest fires thousands of miles away.⁹ The exceptional event rule is a necessary tool for these uncontrollable natural circumstances; however, changing times require changing enforcement.

Air quality nationwide is at risk of returning to unhealthy levels given the exponential increase in natural and human-caused environmental damage.¹⁰ West coast cities, like Portland¹¹ and San Francisco¹² are increasingly subject to multi-day smoke damage from now seasonal forest fires.¹³ Eastern states are similarly impacted by the cross-continental transportation of harmful smoke through the jet stream.¹⁴ Outside the impact of forest fires, some climate experts predict disastrous toxic wind events in cities like Salt Lake City, which sit next to dormant toxic dust bombs in the form of endorheic lakes.¹⁵ These increasingly common “exceptional” events should force the EPA to ask: are events truly “exceptional” under the exceptional event rule if anthropogenic sources lead to recurring events?

This article will seek to answer this question and examine the role the exceptional event rule will play in the EPA’s future enforcement of the CAA. Part II of this article will examine the plain language of the exceptional event rule in the United States Code and in the EPA’s Rules and Regulations. Part III will highlight the current application of the exceptional event rule, demonstrating the oxymoronic relationship between the dichotomous approach and hybrid approach to causation. Part IV will conclude this article with an examination of how the EPA’s approach to causation under the exceptional event rule could fail to account for the significant air quality events of the near future.

II. THE EXCEPTIONAL EVENT RULE 

The plain language of the exceptional event rule provides definitions, regulations, guiding principles and requirements, and interim provisions.¹⁶ An exceptional event can be summarized as an event (1) affecting air quality, (2) in a manner that is “not reasonably controllable or preventable,” (3) caused by a natural event, or human activity that is unlikely to recur at that specific location.¹⁷ The administrator overseeing a request for an exemption under the exceptional event rule must consider:

(i) the principle that protection of public health is the highest priority;
(ii) the principle that timely information should be provided to the public in any case in which the air quality is unhealthy;
(iii) the principle that all ambient air quality data should be included in a timely manner,2 an appropriate Federal air quality database that is accessible to the public;
(iv) the principle that each State must take necessary measures to safeguard public health regardless of the source of the air pollution; and
(v) the principle that air quality data should be carefully screened to ensure that events not likely to recur are represented accurately in all monitoring data and analyses.¹⁸

When assessing these factors, the administrator should only consider a request for exemption when the occurrence of the exceptional event is demonstrated by “reliable, accurate data” produced in a prompt manner.¹⁹ Furthermore, “a clear causal relationship must exist” between the measured violation of NAAQS and the exceptional event alleged.²⁰

Interpreting this statute, the EPA has provided significant literature in order to provide guidance for federal, state, and tribal entities that aim to tap into the exceptional events exemption to NAAQS.²¹ In 2016, the EPA produced a “final rule” with the intent to clarify how events are defined under the exceptional event rule.²² Most notably, the EPA promulgated new language to account for areas with “historically documented” or “known seasonal” exceptional events.²³ However, the EPA only adds that “mitigation plans” must be followed in these high risk areas in order to qualify for the requirements of the exceptional events rule.²⁴ Despite the EPA’s intent to provide clarity on the issue, questions have remained in regard to recurring events.²⁵ Specifically, an event that repeatedly exceeds NAAQS will be exempted so long as it is natural; whereas, an anthropogenic source must be “unlikely to recur at a particular location.”²⁶ The EPA’s enforcement of the rule has raised significant questions when the event is caused by both anthropogenic sources and natural sources; referred to herein as “hybrid-natural” sources.

III. APPLICATION OF THE RULE

Although the exceptional event rule may appear to be a location-restricted exemption, it is an issue that concerns all states. For example, shortly after the EPA produced its “final rule,” Connecticut, Massachusetts, New Jersey, Rhode Island, Maryland, and Pennsylvania were all granted a naturally-caused exceptional event exemption for pollutants originating from Alberta, Canada.²⁷ In 2017, Louisiana was able to successfully receive an exceptional event exemption when Hurricane Irma pulled significant levels of ozone originating from forest fires in the Pacific Northwest.²⁸ The overwhelming natural contributions to theses states’ NAAQS violations fall cleanly within the “natural event” category of the exceptional event rule.²⁹ For states locationally removed from the source of the pollutant, a “natural” causation is appropriate, regardless of the true source of the pollutants, because the event would not occur but for the naturally-caused transportation of pollutants from one location to another.³⁰ It follows then that states like Tennessee, Mississippi, and Louisiana, which have limited incidents of local forest fires and dust events, will often cite natural causation under the exceptional event rule. However, a “hybrid-natural” causation, which is especially common when local entities attempt to invoke the exceptional event rule, is problematically assumed to be natural in cause only. ³¹

The most notable challenge to the EPA’s enforcement of the anthropogenic/natural exceptional event dichotomy was decided in 2018 D.C. United States Court of Appeals case, Natural Resources Defense Council v. EPA.³² At issue was how the EPA regulated the differences between “naturally caused” exceptional events and “human caused” exceptional events.³³ Most offensive to the environmental groups bringing the challenge was that “naturally caused” events included events that occur as a result of both human and natural activity.³⁴ These groups reasoned that an event caused in part by human activity cannot be a natural event, to which the EPA responded that exceptional events often require a discretionary determination as to the causal source.³⁵ This determination is critical to exceptional event exemptions, because a “recurring” event may only be deemed an exceptional event if the event is naturally caused.³⁶ Therefore, a recurring, long-term event, caused by both human and natural activity can give a state or tribal entity an exemption from NAAQS.³⁷ 

As stated by the D.C. Circuit Court, “an event is natural if it resulted from at least some natural activity and any amount, no matter how significant, of reasonably controlled human activity.”³⁸ Thus, environmental groups have argued the naturally caused distinction has become moot in the face of significant events that encompass both causal activities.³⁹ As an example, the environmental groups suggest that a high wind event that blows power plant pollutants resulting in a down-wind NAAQS violation could be deemed “natural” despite the obvious human source of the pollutants.⁴⁰ Ultimately, the Court determined under both Chevron deference and an arbitrary and capricious standard that the EPA’s distinct and sometimes hybrid definition of “natural event” was lawful.⁴¹ Synthesized with the statute, it is now evident that a recurring NAAQS violation will fall under the lower naturally-caused threshold so long as there is even a miniscule natural element impacting causation.

The oxymoronic enforcement of exceptional events rule under hybrid-natural causes is best highlighted by Albuquerque’s 2016 request for exemption after a dust storm placed the city in exceedance of NAAQS.⁴² In the EPA’s concurrence letter and technical review, the EPA considered separately whether the natural and human sources were reasonably controllable.⁴³ As to natural sources, the EPA agreed with Albuquerque’s assessment that a simple demonstration that the high wind threshold was exceeded on undeveloped, arid land would be sufficient to show the contribution was outside the city’s reasonable control.⁴⁴ As to anthropogenic sources, the EPA acknowledged “the dominant source of particulate matter near the South Valley site is anthropogenic.”⁴⁵ Under the heightened anthropogenic cause analysis, the EPA found that agency steps to reasonably control dust was sufficient because the city oversaw and enforced dust permits and dust control measures throughout the anthropogenic source area.⁴⁶ Despite the major contribution from anthropogenic sources, the EPA found that the high wind dust event was “considered a natural event.”⁴⁷ This event highlights that hybrid-natural events are especially easy to prove, even when the primary source of the pollutant is anthropogenic. This is especially concerning because hybrid-natural events are also exempt from consideration of whether the event “is unlikely to recur at a particular location.”⁴⁸

IV. A COMPLEX FUTURE

In summary, the exceptional event rule is at risk of being swallowed by hybrid-natural enforcement. Application of the exceptional event rule has devolved to the point that the EPA exempts controllable anthropogenic sources of NAAQS violations so long as a natural source can be included in the event.⁴⁹ To make matters worse, a recurring, long-term event can be exempted so long as it was caused by natural or hybrid-natural sources.⁵⁰ This type of enforcement could lead to significant long-term exceedance of NAAQS, plummeting ambient air quality to pre-CAA levels.  

Consider the air quality dangers in significant population centers that lie near potentially toxic endorheic lakes, such as Salt Lake City, Reno, and the farming towns and cities of California’s Imperial Valley. Intermittent and significant droughts are contributing to the drying of highly volatile lakes⁵¹ and human-caused irrigation contributes to a significantly high degree in regard to the Salton Sea and Great Salt Lake.⁵² Experts believe that if the Great Salt Lake dries up, as is predicted in the next ten years with current water usage, then Salt Lake City could be plagued by toxic arsenic-laced air year round.⁵³ The Salton Sea, which is plagued by reduced water input from a myriad of factors, has begun kicking up dust laced with lead, chromium, and DDT.⁵⁴ Without reasonable control and interference from state entities, these lakes will dry up, and any sufficient wind event will cause significant NAAQS violations.⁵⁵ These toxic wind events will likely be deemed natural events under the hybrid-natural rule, thus providing a lower threshold for proving an exceptional circumstance beyond reasonable control or prevention.⁵⁶ If the toxic dust storms in Salt Lake City and the Imperial Valley become recurring, then the EPA will likely grant them a long-term naturally-cause exceptional event exemption.⁵⁷ 

In effect, holding these events to be natural would swallow the intent of the CAA and the exceptional event rule. First and foremost, it could undermine the explicit rule that the administrator in charge of determining an exceptional event must consider public health above all other factors.⁵⁸ The concern presented here is that the exceptional event rule will subvert this exact concern if the exemption is granted anytime a recurring windstorm kicks up toxic dust. For the Great Salt Lake and the Salton Sea, human-caused factors are the predominant contributors to these more than likely recurring dust storms, but the exceptional event rule’s liberal hybrid-natural causation rule will absolve state entities from their environmental obligations.⁵⁹ In light of this concern, the EPA must consider whether the current enforcement of the exceptional event rule truly prioritizes public health above all other factors.⁶⁰ As it currently stands, an anthropogenic dominated hybrid-natural ruling cannot be tolerated under the exceptional events rule, which only intends to provide exemption for natural or non-recurring anthropogenic events.⁶¹  

Looking forward, the CAA’s exceptional events rule should be utilized to encourage entities to aggressively curb anthropogenic contribution to hybrid-natural events. The current enforcement scheme of the exceptional event rule incentivizes state entities to take bare-minimum control measures and then pin the blame on any contributory natural factor.⁶² The exceptional event rule is at risk of being swallowed by its enforcement; thus, the EPA must consider whether the scope of the exceptional event rule has exceeded the public health priority of the CAA.⁶³

In summary, the exceptional events rule is a necessary tool to ensure fair enforcement of NAAQS. It ensures that entities are not punished for uncontrollable and unpreventable natural or unlikely anthropogenic events.  However, the hybrid-natural causation rule allows entities to redirect causation to a natural classification. This classification allows entities to claim exceptional events under a recurring event, and the threshold to prove reasonable control is significantly lower. As a result, the exceptional events rule will likely see extreme utility in localized areas with volatile hybrid-natural events. In light of this reality, the EPA must consider whether the current enforcement model of the exceptional event rule prioritizes public health to the extent required. 

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ALTON SMITH is a 3L at the University of Memphis Cecil C. Humphreys School of Law. After graduation, Smith will be a law clerk with Judge John York in the U.S. District Court for the Western District of Tennessee. After completing this clerkship, Smith will join Hinkle Shanor LLP in Santa Fe, New Mexico.

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1. Nadja Popovich, America’s Skies Have Gotten Clearer, but Millions Still Breathe Unhealthy Air, N.Y. TIMES (June 19, 2019), https://www.nytimes.com/interactive/2019/06/19/climate/us-air-pollution-trump.html.
2. Id.; United States: Clean Air Act (1970), AIR QUALITY LIFE INDEX, https://aqli.epic.uchicago.edu/policy-impacts/united-states-clean-air-act/ (last visited April 14, 2023).
3. Popovich, supra note 1; State of the Air 2022, American Lung Association, https://www.lung.org/getmedia/74b3d3d3-88d1-4335-95d8-c4e47d0282c1/sota-2022 11–17 (2022).
4. Treatment of Data Influenced by Exceptional Events, 81 Fed. Reg. 68216-01 (Env’t Prot. Agency Oct. 3, 2016). 
5. 42 U.S.C.A § 7619 (West 2005).
6. Treatment of Data Influenced by Exceptional Events, 81 Fed. Reg. 68216-01 (Env’t Prot. Agency Oct. 3, 2016). 
7. Id.
8. Id. at *68217. 
9. Example Demonstrations and EPA Responses Prepared under the 2016 Exceptional Events Rule, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/example-demonstrations-and-epa-responses-prepared-under-2016-exceptional (last visited April 13, 2023).
10. Popovich, supra note 1; Wildfire Climate Connection, NAT’L OCEANIC AND ATMOSPHERIC ADMIN. (Aug. 8, 2022), https://www.noaa.gov/noaa-wildfire/wildfire-climate-connection.
11. Brian Brennan, Experts Warn of Northwest Future with More Wildfire Smoke, Health Issues, KGW (Aug. 17, 2018), https://www.kgw.com/article/weather/wildfires/experts-warn-of-future-with-more-wildfire-smoke-poor-air-quality/283-584969499.
12. Jack Lee, Wildfire Smoke in the Western U.S. is Reversing Air Quality Gains Over the Past Two Decades, S.F. CHRONICLE (Oct. 17, 2022), https://www.sfchronicle.com/weather/article/Wildfire-smoke-air-quality-17510535.php.
13. Id.
14. Nadja Popovich & Josh Katz, See How Wildfire Smoke Spread Across America, N.Y. TIMES (July, 21, 2022), https://www.nytimes.com/interactive/2021/07/21/climate/wildfire-smoke-map.html; Example Demonstrations and EPA Responses Prepared under the 2016 Exceptional Events Rule, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/example-demonstrations-and-epa-responses-prepared-under-2016-exceptional (last visited April 13, 2023).
15. Margaret Osborne, Drying Great Salt Lake Could Expose Millions to Toxic Arsenic-Laced Dust, SMITHSONIAN MAG. (Jan. 13, 2023), https://www.smithsonianmag.com/smart-news/drying-great-salt-lake-could-expose-millions-to-toxic-arsenic-laced-dust-180981439/ (citing Abbott et al, Emergency Measures Needed to Rescue Great Salt Lake from Collapse, Brigham Young University (Jan. 4, 2023), https://pws.byu.edu/GSL%20report%202023).  
16. 42 U.S.C.A § 7619 (West 2005).
17. 42 U.S.C.A § 7619(b)(1)(A) (West 2005). 
18. 42 U.S.C.A § 7619(b)(3)(A) (West 2005).
19. 42 U.S.C.A § 7619(b)(3)(B)(i) (West 2005).
20. 42 U.S.C.A § 7619(b)(3)(B)(ii) (West 2005).
21. Treatment of Data Influenced by Exceptional Events, 81 Fed. Reg. 68216-01 (Env’t Prot. Agency Oct. 3, 2016).
22. Id. at *68216–18.
23. Id. at *68218.
24. Id. 
25. Natural Res. Def. Council v. Env’t Prot. Agency, 896 F.3d 459 (D.C. 2018).
26. 42 U.S.C.A § 7619(b)(1)(A) (West 2005).
27. Example Demonstrations and EPA Responses Prepared under the 2016 Exceptional Events Rule, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/example-demonstrations-and-epa-responses-prepared-under-2016-exceptional (last visited April 13, 2023).
28. Exceptional Events Documents, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/exceptional-events-documents-ozone-louisiana (last visited April 15, 2023).
29. 42 U.S.C.A § 7619(b)(1)(A) (West 2005). 
30. See Exceptional Events Documents Ozone – Louisiana, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/exceptional-events-documents-ozone-louisiana (last visited April 15, 2023) (where the causes of the fires and the trans-national transportation were both naturally caused exclusively).
31. Natural Res. Def. Council v. Env’t Prot. Agency, 896 F.3d 459, 464–65 (D.C. Cir. 2018).
32. Id.
33. Id. at 463. 
34. Id. at 463–64. 
35. Id. at 464–65.
36. Id.; 42 U.S.C.A § 7619(b)(1)(A) (West 2005). 
37. Natural Res. Def. Council, 896 F.3d at 465–66.
38. Id. at 463.
39. Id.  
40. Id. at 464.
41. Id. at 464–65.
42. Exceptional Events Documents Particulate Matter – Albuquerque, NM, ENV’T PROT. AGENCY, https://www.epa.gov/air-quality-analysis/exceptional-events-documents-particulate-matter-albuquerque-nm (last visited April 15, 2023).
43. Anne L. Idsal, EPA Concurrence Letter and Technical Review for All Three Demonstrations, ENV’T PROT. AGENCY REGION 6, (March 22, 2018), 9–10, https://www.epa.gov/sites/default/files/2019-03/documents/2018_march_epa_letter_to_coa_2016_ee_demo_response.pdf.
44. Id. at 9.
45. Id. at 9–10.
46. Id.
47. Id. at 10.
48. 42 U.S.C.A § 7619(b)(1)(A) (West 2005).
49. Anne L. Idsal, EPA Concurrence Letter and Technical Review for All Three Demonstrations, ENV’T PROT. AGENCY REGION 6 (March 22, 2018), 9–10, https://www.epa.gov/sites/default/files/2019-03/documents/2018_march_epa_letter_to_coa_2016_ee_demo_response.pdf. 
50. Natural Res. Def. Council v. Env’t Prot. Agency, 896 F.3d 459, 465–66 (D.C. Cir. 2018).
51. Jida Wang, et al., Recent Global Declin in Endorheic Basin Water Storages, NATIONAL LIBRARY OF MEDICINE, (Nov. 30, 2018), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6267997/. 
52. Abbott et al, Emergency Measures Needed to Rescue Great Salt Lake from Collapse, Brigham Young University (Jan. 4, 2023), https://pws.byu.edu/GSL%20report%202023.
53. Margaret Osborne, Drying Great Salt Lake Could Expose Millions to Toxic Arsenic-Laced Dust, SMITHSONIAN MAG. (Jan. 13, 2023), https://www.smithsonianmag.com/smart-news/drying-great-salt-lake-could-expose-millions-to-toxic-arsenic-laced-dust-180981439/ (citing Abbott et al, Emergency Measures Needed to Rescue Great Salt Lake from Collapse, Brigham Young University (Jan. 4, 2023), https://pws.byu.edu/GSL%20report%202023).  
54. Chris Iovenko, Toxic Dust From a Dying California Lake, The Atlantic (Nov. 9, 2015) https://www.theatlantic.com/science/archive/2015/11/the-airborne-toxic-lake-event/414888/.
55. Id. (stating, “the human-health costs of a dead, shrunken Salton Sea are enormous. Some voice concern that a severe wind event might spread the lake’s hazardous dust towards the heavily populated coastal cities or even as far as Arizona, affecting millions.”)
56. 42 U.S.C.A § 7619(b)(1)(A) (West 2005).
57. See Natural Res. Def. Council v. Env’t Prot. Agency, 896 F.3d 459, 464–65 (D.C. Cir. 2018).
58. 42 U.S.C.A § 7619(b)(3)(A) (West 2005).
59. See Natural Res. Def. Council, 896 F.3d at 465–66.
60. 42 U.S.C.A § 7619(b)(3)(A) (West 2005).
61. 42 U.S.C.A § 7619(b)(1)(A) (West 2005); Treatment of Data Influenced by Exceptional Events, 81 Fed. Reg. 68216-01, at *68216–18 (Env’t Prot. Agency Oct. 3, 2016).
62. Natural Res. Def. Council, 896 F.3d at 463 (where the environmental groups claimed the natural/anthropogenic dichotomy has become moot, and states could abuse this).
63. 42 U.S.C.A § 7619(b)(3)(A) (West 2005).